The Great Lakes are the largest fresh water system in the world. The basin is home to Canada's largest urban population, which exacts a toll on the lakes. Chemicals and pharmaceuticals are flushed into the lakes, altering the health of all species in the water. We board the Limnos, a floating laboratory used by the National Water Research Institute to monitor water quality, fish populations and invasive species like the zebra mussel. Inland, we look at the impacts of farming on waterways and see what's being done to clean them up. Hundreds of small municipalities are responsible for delivering safe drinking water to their residents, but what if the system fails? We examine the Walkerton tragedy, E-coli outbreaks, widespread illness and death. Can it happen again?